Jérôme Bossuet

The finite phosphorus challenge

Maintaining the fertility of our farmland is vital, if we are to feed our expanding population without compromising the resource base on which future generations will depend. But while the talk at the Rio +20 summit was of 'sustainable intensification', most intensive crop agriculture currently relies on inputs from non-renewable fossil resources, and in the case of one essential nutrient, that resource may not be available to us for much longer.

Phosphorous - the 'P' in NPK fertiliser - is an essential nutrient for plant growth, necessary for building cells and transferring energy. Fertiliser manufacturers source the mineral from phosphate rocks, but these are a very localised resource: most of the world's phosphate reserves are found in just three countries - Morocco, USA and China. And while the rocks took millions of years to form, their future could be alarmingly short; the US is thought to have less than 30 years of reserves left, and China is gradually reducing exports in order to secure its domestic supply.

Current demand for phosphorous from global agriculture totals around 150 million tons per year, and is estimated to rise by 50-100 per cent by 2050. But while the fertiliser industry claims to have enough for a thousand years, others see the end of phosphorous mining within a few decades. Despite this controversy, changing fertilisation practices in favour of more ecological approaches, and addressing the fertiliser gap for farmers in the South who can seldom afford chemical inputs, should both be encouraged.

Sustainable and affordable fertiliser

Smallholder farmers in the Sahel, for example, rarely use fertilisers, because they tend to be either not available or are only sold in 50kg bags, and hence are not affordable. Yields of pearl millet, a major staple crop in West Africa, rarely exceed 900kg per hectare due to the lack of major nutrients in the soil, such as phosphorus.

Packaging fertilisers in smaller packs is one way of improving farmers' access to them, and this has been introduced successfully in Burkina Faso, Mali and Niger by ICRISAT and AGRA. Micro-dosing has also proved valuable, enabling farmers to boost yields with minimal fertiliser usage. Using a bottle cap, farmers apply tiny amounts of fertiliser - one sixth of the quantities normally used on grain crops in Europe - directly to the roots of the plant. Research by ICRISAT showed this increased yields in Niger by 55 per cent, and that micro-dosing has contributed an estimated 70,000 tons of additional grain in drought-prone regions of Africa.

Other fertiliser-saving approaches would be to use phosphorus-coated seeds or to add a micro compost dose to sowing peats, in order to boost the initial growth of the seedlings. But to scale up these practices, partnerships between development stakeholders and the private sector will be needed.

Improving plants' ability to access the phosphorous found naturally in the soil is also worthy of investigation. Sandy soils, exposed to high run-off and evaporation, tend to have low phosphorous levels, and what is present is often in an insoluble form that plants cannot absorb. But in certain soil conditions, legume crops like groundnut, chickpea or pigeonpea can convert this phosphorous into a soluble form, from which subsequent cereal crops can also benefit. This is to be investigated further by the CGIAR through its new, soon-to-be-launched, global Dryland Legume Research Programme.

Recycling phosphorus

Reducing phosphate use is good for the planet; but to end our dependence on this fossil resource means that agriculture has to find alternatives. One option could be adopted anywhere: recycling. Phosphorous can be recycled through the food chain and re-used as fertiliser, either directly or after treatment. Recycling methods include burying crop residues by ploughing, re-using nutrients found in waste water and human faeces, and composting of food waste, both from households and agribusinesses. An estimated 30 per cent of agricultural production is wasted between the field and our plates, including that thrown away uneaten by consumers.

Several issues exist: wastes and faeces are bulky and tend to have low concentrations of phosphorous, and sanitary aspects must obviously be addressed to avoid spreading diseases. Current water and sanitation systems would need to be adapted to separate urine and solid faeces, but this is a field of innovation ripe for development if we want a greener society. Some countries are certainly open to this debate with, for instance, northern Europe encouraging sustainable housing schemes which incorporate ecological (and cheaper) dry latrines. These 'ecosan' latrines are also promoted in developing countries to combat water-borne diseases with a double advantage: less water consumption and clean and easy recycling of wastes as fertiliser for agriculture.

Phosphorus represents one of many sustainability challenges brought up at Rio. We know many "greener" options to NPK-fed agriculture exist but how do we get these used more widely when it seems easier to continue extracting from mines? That's the dilemma of the green economy.